Hi,

I can not get my central heating to work in "heating only" mode. I do not
know if this has ever worked as I just moved house. (House is approx 15
years old and most of central heating components have been replaced in the
past 5 years I believe.)

It works fine in "Hot water only" or "both heating and hot water" but when I
just turn on the central heating at the programmer nothing happens.

The programmer is new (Tempus 7).

I suspected the 3 way valve was faulty so I changed this for a new one,
(Danfoss Randal H3A) Initially I just fitted the new actuator, but then
fitted a new valve with the new actuator. This didn't make a difference.

When the programmer is set in "Heating Only" mode I am able to use the
manual override switch on the 3-port valve and then the heating comes on.
(The water also comes on as the valve is in the mid position when using the
manual override switch.)

I did start to attempt to make a wiring drawing of the whole central heating
electrics, but so far this is proving to be complicated as I can not confirm
exactly which cables are which at the junction box in the airing cupboard.

One thing I did note is that on the programmer only the outputs for Ho****er
On and Heating On are connected. Ho****er Off and Heating Off are not used.
Is this significant?

Any suggestions where to start looking?

Thanks,

Matthew